In the Hall-Heroult process of producing aluminum by the electrolytic reduction of alumina, using a fluoridic molten flux as conductive medium, solid and gaseous effluents are emitted from the reduction cells. These effluents include carbon particles from the anodes employed, volatilized metallic and nonmetallic impurities from the alumina feed and other gaseous compounds generated by the electrolytic process. Strict environmental and safety regulations require capture of these undesirable effluents and a common method employed for offgas purification involves the utilization of dry scrubber systems. Dry scrubber systems presently in use in aluminum reduction plants use alumina as a sorbent for the capture of harmful effluents. The reasons for the use of alumina in these scrubbers are many fold; aside from its well-known sorptive capacity, including its capability of combining with the fluorides of the offgas, the exhausted alumina sorbent can be employed as feed for the reduction cells, thus eliminating a disposal problem. Nevertheless, there are significant problems associated with the recycling of the impurity-laden alumina scavenger as feed to the reduction cells. One of the main problems results from the high metallic impurity level of the alumina scavenger to be charged to the cells. Typical metallic impurities, which have been captured by the alumina in the dry scrubber system include iron, nickel and vanadium; also significant quantities of magnesium, calcium and sodium will be retained in the alumina together with phosphor pentoxide. The major nonmetallic impurity which contaminates the alumina of the dry scrubber system is carbon which results from the degradation and incomplete burning of the carbon electrodes, such as prebaked or Soederberg anodes, commonly employed in the reduction cells. Return of these impurities to the cells with the spent alumina scavenger enriches the produced aluminum metal in undesired metallic and nonmetallic impurities; the high carbon content of the feed may cause additional feeding problems, such as sudden combustion of the carbon in the cell at the high operating temperatures; and the current efficiency of the cell is considerably reduced.
To overcome these undesirable effects it has been suggested to employ a fluidized bed reactor containing alumina for the removal of fluoridic and other impurities from the cell offgases. The partially purified offgases, containing entrained alumina particles, are then introduced into an electrostatic precipitator where the offgas purification is completed and where simultaneously grain size classification of the impurity-laden alumina, entrained in the offgas, takes place. The fine fraction obtained by the electrostatic precipitation, which has been found to contain most of the undesirable impurities, is subjected to pyrohydrolysis to remove its fluoride content, while the coarse fraction is recycled to the cell as feed after a thermal treatment to remove its HF content. The HF-free alumina which is obtained after pyrohydrolysis is still highly contaminated in noncombustible and nonvolatilizable metallic and nonmetallic impurities and is unsuitable for recycle to the reduction cells and thus poses a problem of disposal. While the above-described system avoids reintroduction of significant quantities of impurities to the reduction cells, it requires close control of operations, complicated mechanical and electrical equipment and still results in a large volume of impure alumina which cannot be utilized as feed for the reduction cells.
It has now been discovered that these disadvantages of the prior art systems can be readily resolved and eliminated by employing an efficient and simple method for the purification of the impurity-laden alumina obtained in the dry scrubbing of aluminum reduction cell offgases. The instant process utilizes an ultrasonic treatment of a slurry of the impure alumina which will remove the impurities from the alumina scavenger and allows the return of all of the purified alumina to the reduction cells without requiring additional cleaning and without increasing the impurity level of the produced aluminum metal in a significant degree.